1. Field of the Invention
This invention relates to a crossed-field amplifier and, more particularly, to an electron emitting device used within a crossed-field amplifier to reduce amplifier jitter caused by stopping and restarting the amplifier.
2. Description of Related Art
Crossed-field amplifiers (CFAs) have been used for several years in electronic systems that require high RF power, such as radar systems. A CFA operates by passing an RF signal through a high voltage electric field formed between a cathode and an anode. The cathode emits electrons which interact with an RF wave as it travels through a slow-wave path provided in the anode structure surrounding the cathode. The RF wave is guided by a magnetic field, which crosses the electric field perpendicularly. Crossed-field amplifiers are disclosed in U.S. Pat. No. 4,700,109 issued Oct. 13, 1987, to MacPhail, and U.S. Pat. No. 4,814,720, issued Mar. 21, 1989, to MacPhail et al., both assigned to the common assignee, and which are incorporated herein by reference.
In some applications, it is desirable to operate the CFA in a pulsed mode in which the CFA is repeatedly turned on and off. If used in a radar system, accuracy of the pulse timing is critical to obtaining accurate return information. To start a CFA, there must exist a small number of electrons in the interaction region in order to prime the operation of the cathode. These priming electrons come from natural sources, such as residual radioactivity, electron storage from preceding pulses, cosmic rays, etc. The priming electrons impact the cathode structure causing secondary emissions of electrons from the cathode surface, further resulting in a cascade of electrons flowing in a beam through the interaction region. At relatively high pulse repetition frequencies, a large number of electrons remain in the interaction region after the CFA has been turned off. These remaining electrons prime the CFA to rapidly restart the secondary emission process. However, at low pulse repetition frequencies the electrons in the interaction region dissipate into the anode structure, leaving an absence of electrons to prime the CFA upon restart. Although the natural source electrons will eventually start the CFA, the start-up time can not be determined with certainty. Thus, the restart of the CFA at low pulse repetition frequencies is highly irregular, and is a phenomenon known as "jitter."
Solutions to the jitter problem have centered on maintaining a supply of electrons in the interaction region during the period in which the CFA is turned off. One such solution involves the use of a bias circuit which holds the electrons in the interaction region between the cathode and the anode when the CFA is turned off. The bias circuit is disclosed in U.S. Pat. No. 4,894,586, issued Jan. 16, 1990, by Crager et al., which is assigned to the common assignee. The bias circuit supplies a negative DC voltage to the cathode which holds the electrons within the interaction region. A significant drawback of this method is that a power supply and transformer are required to supply and regulate the DC voltage. The addition of the power supply increases the complexity of the CFA, and the DC voltage must be insulated from the cathode pulse voltage, which is typically more than 10,000 volts.
Thus, a solution to the jitter problem is sought that does not rely on external power sources, or in which the external power source is displaced away from the cathode.